Fully-Integrated Low-Dropout Regulators

Table of Contents

1. Frontmatter

2. Chapter 1. Introduction to the Low-Dropout Regulator

   Xiangyu Mao, Yan Lu, Rui P. Martins

   Abstract

   This chapter introduces the common definitions and several typical application scenarios of fully integrated low-dropout regulators (LDOs). We classify fully integrated LDO into analog LDO, digital LDO, and switching LDO according to the control method of the power transistors. The goal is to provide the readers a general understanding of the LDO circuits and LDO in systems.

3. Chapter 2. LDO Specifications

   Xiangyu Mao, Yan Lu, Rui P. Martins

   Abstract

   The specifications of LDO provide guidelines and directions for circuit design. This chapter introduces basically all the LDO specification indicators and several figures of merit (FoMs) that are commonly used in academic papers, including the DC, AC, transient indicators, and other essential specifications. Some indicators, such as load regulation with loop gain and transient response with loop bandwidth, are interrelated. The importance of these indicators varies depending on the application, with trade-offs. Understanding these definitions allows us to quantify the actual core requirements of the load and select the appropriate design structure.

4. Chapter 3. Analog LDO

   Xiangyu Mao, Yan Lu, Rui P. Martins

   Abstract

   This chapter mainly introduces the core circuits and related classic structures of analog LDO, including single-stage op amps, error amplifiers, compensation technology, buffer, adaptive biasing, etc. For the buffer circuit, we focus on common-source buffers and source follower buffers, including the classic diode-connected common source buffer and super source follower (SSF) structure. In the compensation circuit section, we focus on Miller compensation, nested Miller compensation, damping-factor-control frequency compensation (DFCFC), and dynamic pole-zero compensation. In the biasing section, we introduce adaptive biasing technology and ultralow-power LDO design examples. For the PSR techniques, we analyze the PSR characteristics of output-pole-dominant LDO and internal-pole-dominant LDO and introduce three feedforward ripple cancellation (FFRC) technologies. Flipped-voltage follower (FVF) LDOs are the most commonly used fully integrated LDO architecture. Thus, we introduce a variety of FVF LDO structures and their compensation methods. Finally, we introduce several classic NMOS LDO design cases.

5. Chapter 4. Digital LDO

   Xiangyu Mao, Yan Lu, Rui P. Martins

   Abstract

   This chapter mainly introduces the representative digital LDO (DLDO) structures, including shift register-based DLDO, coarse-fine-tuning DLDO, ADC-based DLDO, event-driven DLDO, computational DLDO, and digital-analog hybrid LDO. In the shift register-based DLDO section, we introduce the classic structure, adaptive frequency technique, and successive approximation recursive (SAR) DLDO. For the coarse-fine-tuning DLDO, we discuss its mechanism of balancing accuracy and transient response and introduce several classic examples. In ADC-based DLDO, we discuss voltage-domain quantizer, time-domain quantizer, and proportional-integral-derivative (PID) controller. In the event-driven DLDO section, we introduce the differences and advantages of event-driven and time-driven and discuss the challenges and corresponding technical points in the design of event-driven DLDO. Next, we introduce two computational DLDO control schemes: the charge and discharge algorithm, and the time-based exponential control. In the digital-analog hybrid DLDO section, we introduce four control schemes: passive analog-assisted (AA), active AA, digital-assisted, and analog-digital merged. Finally, we discuss the stability and reliability issues in advanced processes and large voltage difference conditions, with some solutions.

6. Chapter 5. Switching LDO

   Xiangyu Mao, Yan Lu, Rui P. Martins

   Abstract

   This chapter first discusses the working principle, equivalent model, and ripple analysis of switching LDO. Then, the classic hysteresis switching control LDO structure is introduced. While hysteresis switching control has a fast transient response, it results in a significant output ripple. Increasing the operating frequency and dynamically adjusting the power transistor strength can help reduce this ripple. Multiphase PWM control can effectively decrease the output ripple and reduce the output capacitor requirement. We cover multiphase PWM control circuits, including high-speed continuous comparators, ramp generation circuits, current balancing analysis, and both dual-loop and single-loop control methods. In addition, for digital LDO, part of the switches operating in switching mode can reduce the output ripple and extend the load current range. Finally, we present LDO examples utilizing switching assistance schemes.

7. Chapter 6. Distributed LDO

   Xiangyu Mao, Yan Lu, Rui P. Martins

   Abstract

   Distributed LDOs can effectively mitigate the global IR drop and improve the local transient performances for a high-current large-area power delivery network. However, compared to traditional standalone LDO designs, they face integration, current sharing, and stability challenges. In this chapter, we discuss and analyze these issues in detail and introduce two distributed LDO integration solutions: on-side integration and embedded integration. In addition, we focus on three distributed LDO control solutions: parallel distributed LDO, neighbor cooperative distributed LDO, and dual-loop distributed LDO. For each type, we provide an overview of the architecture and core circuit design.

8. Chapter 7. Conclusions on Fully Integrated LDOs

   Xiangyu Mao, Yan Lu, Rui P. Martins

   Abstract

   This chapter mainly summarizes and discusses the LDO application scenarios, control schemes, and future research directions. LDO is suitable for a variety of application scenarios. In different applications, the specifications of LDO have different weights. The three control schemes (analog, digital, switching) of LDO also have their own advantages and applicable scenarios. Analog control excels in handling small current fast transients and scenarios demanding low noise and high power supply rejection (PSR). Digital control is ideal for managing large signals and facilitating process scalability, while switching techniques are optimal for high-current fast transient digital loads. Hybrid control can combine the advantages of different control schemes in certain scenarios. Future research directions include the development of ultralow quiescent current LDOs, low-noise ultrahigh PSR LDOs, scalable fully synthesizable all-digital LDOs, and AI-based automated LDO designs.

9. Backmatter